Conventionally, ignition timing control apparatus have been known which operates to change ignition timing of an internal combustion engine in accordance with the opening degree of a throttle valve which adjusts the amount of intake air sucked into the engine. In such ignition timing control apparatus, ignition timing is controlled in proportion to the change rate of the opening degree of a throttle valve as shown in FIG. 1. Specifically, as shown in FIG. 1(a), as the throttle opening changes from the closed state toward the fully open state, ignition timing is controlled to advance as shown in FIG. 1(b). On the other hand, as the throttle opening changes from the fully open state toward the closed state, ignition timing is controlled to retard in proportion to the rate of change of the throttle opening.
With a marine vehicle, for example, equipped with such an ignition timing control apparatus, however, brake is often applied to suddenly decelerate the vehicle by changing the engine operation from a forward drive into a rearward drive so as to make the propellant screw blade revolve in a reverse direction. In this case, the engine is subjected to an abrupt increase in load because the screw blade is brought into the rearward drive in spite of the marine vehicle still remaining in a forward driving state.
On the other hand, upon such a change in the drive condition of the engine, the opening degree of the throttle is normally in a fully closed state and ignition timing is thus at the most retarded piston position, so the engine is easily caused to stall due to an abrupt change in the load on the engine.
The present invention is proposed to solve the above-described problems, and has for its object the provision of an ignition timing control apparatus which is able to prevent a marine engine from being stalled upon rapid decelerations of a marine vehicle.